The primary objective of this multi-center sub-study of USPHS Study 23 (Research in the Use of Rifabutin for the Treatment of Active Tuberculosis) is to determine the proportion of patients with HIV-related TB who have abnormal pharmacokinetics of INH and rifabutin. There are two concerns regarding rifabutin and INH pharmacokinetics in this population: 1) malabsorption of anti-TB medications is frequent in this population and 2) there are potential interactions among the medications used in the management of HIV-infected individuals. Correlation of the pharmacokinetic and clinical outcomes in the setting of these interactions is essential. This study will seek to enroll every eligible patient enrolled in Study 23 and receiving study therapy. The only exclusion is a hematocrit of less than 25%. The study will be done on the General Clinical Research Center (GCRC). No one who is suspected of being infectious or is infectious from TB will be enrolled on the GCRC. After informed consent has been obtained, blood will be drawn just prior to the patient=s anti-TB medication dose. Blood will be drawn at 1, 2, 3, 6, and 24 hours post dose. The plasma will be frozen and batch-shipped to a central lab which will perform the pharmacokinetic testing. Patients will be interviewed regarding concomitant medications, gastrointestinal symptoms, and meals relative to study drug dosing.